1. Field of the Invention
The invention relates generally to the field of digital radio signal communications. More particularly, the invention relates to transmitting multiple formed signal beams with diversity to another radio using multiple transmit antenna arrays.
2. Description of the Related Art
Radio communications are sensitive to the environment in which the radio signals are transmitted and received. Communications can be lost or degraded when the environment is not favorable to the type of signals being transmitted. One particular environmental problem is fading due to scattering. This problem can cause multipath interference or completely prevent reception of a transmitted signal. One known way to mitigate fading due to scattering is to transmit the signal with diversity.
Spatial transmit diversity typically uses two antennas spaced apart some distance from each other. The same or different signals are sent from each antenna and the receiver will, in some way, use the signals from both antennas. Many different schemes can be used to transmit and receive the signals. Examples of transmit diversity schemes are transmit delay diversity, space time block coding, and closed loop transmit diversity.
Spatial division systems, such as SDMA (Spatial Division Multiple Access) systems, use beam forming to minimize interference by directing a signal at the particular radio intended for the signal. In some instances, it is also possible to direct nulls toward other users, further reducing interference for those other users. By directing the signal beam using arrayed antennas, the energy in the radio environment can be more precisely concentrated on a single radio receiver, reducing interference with and lowering the noise threshold for other user terminals.